Convincing magistrates to cut custodial sentences is key to keeping the prison population under control, according to the man in charge of the country's prisons.

Martin Narey, now chief executive of the National Offender Management Service, told an audience at the University of Birmingham's School of Law that would, in turn, help reduce re-offending.

He said that unless the trend was reversed none of the Home Office's other efforts to cut re-offending would work.

He said the fines system had collapsed, and the increase in custodial sentencing by magistrates over the past ten years had been dramatic.

"In 1992, 1,600 people were jailed for shoplifting by magistrates," he said. "In 2002, 15,330 people were sent to prison for shoplifting. When I read the statistics I got the statisticians to check over and over again because I didn't believe them," he said.

He said there were also significant geographical disparities in the numbers of custodial sentences given. Magistrates in Dorset, for example, were twice as likely to punish offenders with a custodial sentence. Turning this tide was "very much a priority," he said, and added that he had already spoken to between 4,000 and 5,000 magistrates on the issue over the past 12 months.

"There are 75,000 people in prison and I've got enough space for 80,000. If it grows much higher there will be a lot of people who don't need to be there," he said.

He said expanding basic skills education and making offenders more employable was top of the agenda.

More controversially, ' contestability' for the running of some underperforming prisons would be resumed shortly where the private sector would be invited to bid for contracts, he added.

"This is not privatisation. There are no quotas about the amount of work to go to the private sector. The private sector can and do run good, cost effective prisons."